Thermoplastic foams made from styrenic polymers, such as polystyrene, have found extensive use, particularly as insulating materials. Generally, insulating styrenic foams are produced in thicknesses greater than one-half inch. The insulating value of such foams is measured in terms of heat conduction resistance or R-value, per one inch of foam thickness, and adequate insulating foams typically have R-values of about 5.0 per inch or greater. Styrenic insulating foams to meet government construction standards generally must also be dimensionally stable, i.e., they must have a maximum change in any of length, width or thickness of less than about 2.0 percent when subjected to a 158.degree. F. temperature for 24 hours.
These styrenic foams typically are made by mixing a volatile blowing agent with the styrenic resin under a controlled temperature and pressure sufficient to liquefy and plasticize the resin and to maintain the resin in an unfoamed state, and then extruding the resin-blowing agent mixture through a die into a zone of lower temperature and pressure which results in the formation of the styrenic foam. Blowing agent compositions for styrenic foam production generally are required first to have a system vapor pressure at the melt temperature of the styrenic resin used sufficient to produce acceptable insulating foam, and second to provide sufficient plasticization to the styrenic resin to permit extrusion at commercial production rates. The blowing agent should also not have too great of a plasticizing effect on the styrenic resin, to avoid dimensional instability of the foam.
Presently, production of dimensionally stable, insulating styrenic foams which retain an R-value above 5.0 per inch upon aging, in general requires the use of a blowing agent of certain chlorofluorocarbons, such as dichlorodifluoromethane (Freon-12). Freon-12 has found extensive use with styrenic foams because it meets the vapor pressure and plasticization requirements for a styrenic foam blowing agent. These chlorofluorocarbons are suspected of reacting with ozone after their release into the earth's atmosphere. Thus, alternative styrenic foam blowing agents capable of producing stable, insulating styrenic foams of adequate R-value are very desirable.
Blowing agents for styrenic foams have previously been disclosed. Suh, U.S. Pat. No. 4,438,224 discloses a mixture of certain chlorofluoromethanes and certain lower alcohols. Suh further discusses using a mixture of methylchloride plus chlorofluorocarbons as the blowing agent to produce styrenic foam.
Nakamura, U.S. Pat. No. 3,960,792 discloses a method for producing a styrenic foam using a blowing agent mixture having a specific diffusion rate through the styrenic resin. One particular embodiment disclosed is use of a blowing agent mixture of 30 parts methylchloride, 30 parts neopentane and 40 parts dichlorodifluoromethane. Nakamura also discloses that aliphatic hydrocarbons, including propane, can be used in his method.
Canadian Patent 1,086,450, issued Sep. 23, 1980, claims a styrenic foam having specified characteristics, which is produced using a low permeability blowing agent of a specified formula, such as 1-chloro-1,1-difluoroethane (also known as "FC-142b" which for convenience will be used hereafter). The blowing agent is also disclosed as including a mixture of the low permeability blowing agent with at least one of fluorochloromethane, methylchloride, ethylchloride, chlorodifluoromethane, or 1,1-difluoroethane.
Suh, U.S. Pat. No. 4,636,527 discloses a process for the preparation of an alkenyl aromatic foam, such as a polystyrene foam, using a blowing agent mixture comprising about 3 to 45 wt. % carbon dioxide, about 5 to 97 wt. % ethylchloride and from about 0 to 90 wt. % of a fluorocarbon member which is dichlorodifluoromethane, 1-chloro-1,1-difluoroethane or a mixture of the two chlorofluorocarbons.
Akiyama, U.S. Pat. No. 4,451,417, discloses a polystyrene foam extrusion process using as a blowing agent a mixture of 50 to 80 wt. % dichlorodifluoromethane and 20 to 50 wt. % ethylchloride.
Propane has been used commercially as a blowing agent for making thin polystyrene foams having thicknesses less than one-half inch. However, to Applicant's knowledge, propane has not previously been used commercially in production of insulating styrenic foams having thicknesses above about one-half inch.
Blowing agent mixtures have also been used to produce foams from resins other than styrenic resins. For example, Knaus, U.S. Pat. No. 4,308,352 discloses a process for the production of a polysulfone foam using a blowing agent of either a mixture of methylchloride plus ethylchloride plus 1-chloro-1,1-difluoroethane or a mixture of methylchloride plus propane as the blowing agent to produce such a polysulfone foam.
Park, U.S. Pat. No. 4,528,300 discloses a process for producing a polyolefin foam employing a blowing agent comprising 50 to 95 wt. % of FC-142b plus 5 to 50 wt. % of an aliphatic hydrocarbon and/or a halogenated hydrocarbon having a boiling point from 0.degree. to 50.degree. C. Specifically disclosed examples of the aliphatic hydrocarbon and/or halogenated hydrocarbon are ethylchloride and butane. The method disclosed in Park also requires the presence of a stability control agent such as stearamide in the extrusion mass comprising the polyolefin resin and the blowing agent mixture to produce a stable polyolefin foam. Park does not disclose use of his blowing agent to produce styrenic foams and does not disclose the use of propane, which has a boiling point of -42.1.degree. C., in his blowing agent mixture.
Park, U.S. Pat. No. 4,640,933 also discloses a process for producing a polyolefin foam which employs the use of a blowing agent mixture comprising, for example, isobutane plus FC-142b, or isobutane plus ethylchloride, along with the stability control agent disclosed in U.S. Pat. No. 4,528,300. Park also does not disclose use of his blowing agent to produce styrenic foams nor does he disclose the use of propane in his blowing agent mixture.
Dill, U.S. Pat. No. 3,640,916 discloses a mixture of butane and FC-142b for use as a propellant. Dill is not directed to the use of such a mixture as a styrenic foam blowing agent.
None of the prior art styrenic foam processes have disclosed a blowing agent system of one or more of a certain group of halogenated ethanes, such as FC-142b, with ethylchloride and propane. It is an object of the invention to provide a styrenic foam blowing agent having a reduced ozone reactivity potential, which comprises such components. It is yet another object to produce a dimensionally stable, insulating styrenic foam having an R-value per inch above about 5 using the new blowing agent. It is also an object to provide an improved insulating styrenic foam extrusion process. Other objects will be apparent from the specification.
I have found that the objects of the invention can be attained by the use of a blowing agent composition comprising: at least one halogenated ethane selected from the group consisting of FC-142b, 1,1,1-trifluoro-2-fluoroethane (hereafter referred to as "Freon 134a"), 1-chloro-1,1-difluoro-2,2,2-trifluoroethane (hereafter referred to as "Freon 124"); ethyl chloride and propane. Each of the halogenated ethanes is of less ozone reactivity potential than Freon 12. Each provides sufficient plasticization and has sufficient vapor pressure to produce acceptable insulating foams from polystyrene at commercial production rates. The ethylchloride provides additional plasticization to the molten styrenic resin and the propane lowers foam density. Other benefits of the invention will be addressed below.